1. Technical Field of the Invention
The present invention relates to a fixing device for fixing a toner image onto a sheet, and more particularly, to a fixing device which is provided with a pair of fixing rollers.
2. Description of Related Art
An electrophotographic copying machine and printer are provided with a fixing device for fixing a toner image transferred from a photoconductor onto a sheet. Generally, the machine which is provided with a pair of fixing rollers is known. A heater is housed at least in one of the pair of fixing rollers, wherein a toner image on a sheet which passes through a nip section between the pair of rollers is heated and fused by the heater and fixed onto the sheet.
In such a conventional fixing device, a mold releasing agent coating structure for coating a mold releasing agent or offset-preventive agent on the fixing rollers is provided to prevent toner from being offset to the fixing rollers. The mold releasing agent coating structure is provided with a coating roller for coating the mold releasing agent on the fixing rollers. The coating roller is movably provided toward and away from the fixing rollers in order to avoid wasteful consumption of the mold releasing agent. However, it sometimes happens that an excessive mold releasing agent is coated on the fixing rollers when the coating roller is brought in contact with the fixing rollers even if the coating roller is arranged to be movable toward and away from the fixing rollers.
On the other hand, when a light transmissible colored toner image is fixed onto a transparent film (hereinafter called as OHP film) which is generally used as an original for overhead projectors, it is necessary to fully raise light transmissibility of a fixed image which necessitate to smooth the surface of the fixing rollers as much as possible. Accordingly, in the copying machines and printers which use a light transmissible colored toner, fixing rollers whose surfaces are substantially as smooth as mirror plane are used.
Incidentally, a member such as a thermistor for detecting a temperature is brought in contact with the surface of the fixing rollers. When such a member is brought in contact with the fixing rollers whose surfaces are substantially as smooth as mirror plane, an abnormal sound is generated and/or the fixing rollers are damaged by a friction between the fixing rollers and the member when the mold releasing agent is insufficient. Such troubles easily occur when the fixing device is warmed up immediately after the power is turned on.
In U.S. Pat. No. 4,549,803, there is disclosed a fixing device which is arranged to decrease a rotational speed of fixing rollers less than an ordinary operation when a colored toner is fixed on an OHP film. This is because the toner has to be fully fused when the colored toner is fixed onto the OHP film.
In an electrophotographic copying machine and printer, a sheet is transported at a speed equal to the rotational speed of a photoconductor when a toner image is transferred onto the sheet from the photoconductor. At this time, fixing rollers are also rotated at the same speed as the sheet is transported. Then, the rotational speed of the fixing rollers is changed immediately before the sheet reaches the fixing rollers.
When an OHP film is used, however, trouble sometimes occurs without having the OHP film thrusting into the nip section between a pair of rollers. After thorough study on the cause of the trouble, it was found that the OHP film slips on the fixing rollers. Such a trouble easily occurs especially when a rotating action of the fixing rollers is not stabilized immediately after rotational speed of the fixing rollers is changed. The OHP film tends to be obstructed when it thrusts into the fixing rollers until the rotational speed of the fixing rollers is sufficiently lowered.